This invention concerns a pyroelectric infrared detector and, more specifically, an infrared detector using polymeric pyroelectric elements.
It is well known that certain types of polymer such as polyvinylidene fluoride, polyvinyl fluoride and the like show both pyroelectric and piezoelectric properties when subjected to poling procession and, due to their easy processability, wide variety of application uses are considered.
Infrared detectors constituted with these polymers as pyroelectric elements are, however, defective in that electric signals produced from piezoelectric effect due to mechanical actions such as vibration or bending are much greater than electric signals produced from pyroelectric effect due to incident infrared rays and S/N ratio as the infrared detector is very poor as compared with detectors using ferroelectric ceramics as pyroelectric elements. In order to overcome such defects, a detector using a pair of elements overlapped and connected to each other at their electrodes of same polarities so that the piezoelectricity caused by the bending of the elements may be offset are disclosed, for example, in Japanese Patent Laying Open No. 99869/1977, but the piezoelectricity resulted from the strain in the elements along the thickness can not be offset and the S/N ratio for the electric signals obtained from the infrared detector is not yet satisfactory.
An object of this invention is to provide a pyroelectric infrared detector in which signal to noise ratio in the electric signals can be improved significantly.
Another object of this invention is to provide a pyroelectric infrared detector having a highly excellent detection sensitivity for infrared irradiation.
A further object of this invention is to provide a pyroelectric infrared detector which can reduce electric signals resulted from piezoelectric effect due to mechanical actions applied thereto and process a satisfactory S/N ratio even placed under the conditions where many vibrations are present.
A further object of this invention is to provide a pyroelectric infrared detector which can offset electric signals resulted from pyroelectric effect due to fluctuations in the atmospheric temperature and electric signals resulted from piezoelectric effect due to compression strains respectively.
A still further object of this invention is to provide a pyroelectric infrared detector which has a good processability and can be formed in a small size.
In accordance with this invention, a pyroelectric infrared detector is provided which comprises a substrate and polymeric pyroelectric elements each having electrode plates on both surfaces thereof and mounted to both surfaces of the substrate and in which electrode plates on one of the pyroelectric elements are connected electrically to the electrode plates of opposite polarities on the other of the pyroelectric elements, the thickness of the substrate is greater than the thickness of the pyroelectric elements and infrared rays are adapted to irradiate on one of the pyroelectric elements.
The substrate having a thickness greater than that of the pyroelectric element is, desirably, formed so that the product of the modulus of elasticity and the thickness is great. In preferred embodiments, the substrate can be formed so that its product of the modulus of elasticity and the thickness is greater than that of the pyroelectric element by a factor of 5, preferably, 10 and, more preferably, 20. Materials usable herein for the substrate can include, for example, metal, glass, ceramics, plastics or rubber, and the substrate is preferably prepared from materials having modulus of elasticity greater than that of the polymeric pyroelectric element.
The polymeric pyroelectric element having electrode plates provided on both surfaces thereof can be formed by orienting and polarizing a polymer film or membrane made of a homopolymer such as polyvinylidene fluoride or polyvinyl fluoride, a copolymer comprising vinylidene fluoride or vinyl fluoride as a main component, or a polymer blend comprising either of the above homopolymer or copolymer as a main component. In a preferred embodiment, the film- or membrane-like pyroelectric element is preferably formed to a thickness of 1 to 100 .mu.m and more preferably 2 to 50 .mu.m.
Materials for the electrode plates provided to both surfaces to the polymeric pyroelectric element usable herein include gold, silver, nickel-chromium alloy, aluminum or carbon and the electrode plates are formed through vapor deposition of these materials on the pyroelectric element or through bonding of a film or membrane made of these materials on the pyroelectric element. In a preferred embodiment, the electrode plates are formed to a thickness approximately equal to or less than the thickness of the pyroelectric element. In a further preferred embodiment, the electrode plates are preferably formed to a thickness of 10 A to 2000 A, and more preferably 50 A to 1000 A so that the heat generated by infrared irradiation is preferably conducted to the pyroelectric element. In another preferred embodiment, the electrode plate used as the incident surface of the infrared radiation can be prepared from materials transmittable to infrared rays, for example, a transparent material so that the irradiated infrared rays can transmit it and directly reach the pyroelectric element.
In the electrode plates thus formed, the electrode plates on one of the polymeric pyroelectric elements are connected electrically to the electrode plates of opposite polarities on the other of the pyroelectric elements, for example, by way of lead wires. The electrode plate of the opposite polarity means herein, for example, an electrode plate on which electric charges such as positive or negative charges different from those on the other electrode plate are induced due to pyroelectric when infrared rays are irradiated on both of the pyroelectric elements to result in temperature increase in the same direction in both of the pyroelectric elements and thus produce pyroelectricity in each of the pyroelectric elements.
In a preferred embodiment of this invention, polymeric pyroelectric elements are disposed so that the electrode plates of each of the polymeric pyroelectric elements contacting to the surfaces of the substrate show the polarities opposite to each other. By disposing each of the pyroelectric elements in such a manner, the electrode plates on the sides contacting to the surfaces of the substrate can be set in common to each of the pyroelectric elements, which, as the result, enables to use a substrate prepared by vapor depositing or bonding a conductive membrane or layer as the electrode plate over the entire outer surface or a substrate made of a conductive material also serving as the electrode plate. This can save lead wires for connection. In another preferred embodiment, each of the elements is mounted to the substrate so that the electrode plates on each of the pyroelectric elements contacting to the surfaces of the substrate are on the same polarity.
An infrared detector is constituted by adapting to irradiate infrared rays on one of the pyroelectric elements mounted to both surfaces of the substrate. The incident infrared rays may be focussed by way of a lens or specific spectrum components therein may be removed through color filters. Moreover, infrared rays may be irradiated merely through a transparent glass plate or irradiated directly.